Ultrastructure of cell trafficking pathways and coronavirus: how to recognise the wolf amongst the sheep

Abstract

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has resulted in an urgent need to understand the pathophysiology of SARS-CoV2 infection, to assist in the identification of treatment strategies. Viral tissue tropism is an active area of investigation, one approach to which is identification of virus within tissues by electron microscopy of post-mortem and surgical specimens. Most diagnostic histopathologists have limited understanding of the ultrastructural features of normal cell trafficking pathways, which can resemble intra-and extracellular coronavirus; in addition, viral replication pathways make use of these trafficking pathways. Herein, we review these pathways and their ultrastructural appearances, with emphasis on structures which may be confused with coronavirus. In particular, we draw attention to the fact that, when using routine fixation and processing, the typical 'crown' that characterises a coronavirus is not readily identified on intracellular virions, which are located in membrane-bound vacuoles. In addition, the viral nucleocapsid is seen as black dots within the virion and is more discriminatory in differentiating virions from other cellular structures. The identification of the viral replication organelle, a collection of membranous structures (convoluted membranes) seen at a relatively low scanning power, may help to draw attention to infected cells, which can be sparse.

Ultrastructural modifications induced by SARS-CoV-2 in Vero cells: a kinetic analysis of viral factory formation, viral particle morphogenesis and virion release

“…As previously reported for coronaviruses and, more recently, for SARS-CoV-2 [20][21][22][23], our findings confirm that the SARS-CoV-2 viral cycle is supported by a reticulovesicular network derived from endoplasmic reticulum (ER) membranes and consisting mostly of double-membrane vesicles (DMVs). After the budding of the virions, presumably in the Golgi apparatus stacks or the ER-Golgi intermediate compartment (ERGIC), the virus is released by exocytosis at the plasma membrane.…”

“…At later stages of infection, we observed large intracellular vacuoles containing viral particles that appeared to be completely devoid of spikes. Such images have been reported by other groups, for SARS-CoV-2 propagated in cell culture [22,23,27,33] or detected in the lung tissue of infected patients [34]. Similar findings were also reported in the very first electron microscopy investigations of SARS-CoV in cultured Vero cells [35,36].…”

Hunting coronavirus by transmission electron microscopy – a guide to SARS‐CoV‐2‐associated ultrastructural pathology in COVID‐19 tissues

“…Physiological structures including coated vesicles, multivesicular bodies and cross‐sections of the rough ER are morphological lookalikes of genuine coronaviruses 105 …”

“…Physiological structures including coated vesicles, multivesicular bodies and cross-sections of the rough ER are morphological lookalikes of genuine coronaviruses. 105 Coated vesicles (CV) are single membrane-bound vesicles of variable size (typically 50-150 nm) characterised by 'spiny adornments on their limiting membrane' (Ghadially 106 ) ( Figure 4E). They are involved in endocytosis and membrane trafficking (reviewed in Robinson 107 ).…”

Advanced microscopy technologies enable rapid response toSARS‐CoV‐2 pandemic

“…However, common cellular structures such as clathrin-coated pits, multivesicular bodies containing several intraluminal vesicles and ribosome-decorated ER membranes can be misinterpreted as SARS-CoV-2 virions (Hopfer et al, 2021). Additionally, poor ultrastructure preservation due to autolysis of autoptic samples can lead to ambiguous identification of viral structures (Hopfer et al, 2021;Neil et al, 2020). Thus, controversies arose concerning the correct interpretation of cellular architecture and viral-associated structures in several articles reporting EM images obtained from COVID-19 patient samples (Dittmayer et al, 2020;Hopfer et al, 2021;Miller & Goldsmith, 2020).…”